It is known to cure coatings based on acrylates by radiation, especially by UV radiation. However, the problem occurs here that the presence of air is disruptive in the full curing of the coating material.
DE-C 26 25 538 discloses photopolymerizable coating compositions which include amines as reducing agents and/or chain-transferring compounds.
By incorporating the amino group into a molecule which also contains polymerizable groups, an amine is obtained which is incorporated by polymerization in the course of curing.
A known method of incorporation is the addition of amines onto molecules rich in double bonds, a reaction which takes place by analogy with a Michael addition and which is described, for example, in F. Moller, Houben-Weyl, Vol. 11/1 (1957), pages 277 to 280. U.S. Pat. No. 2,759,913 teaches the addition of amines onto activated, olefinically unsaturated compounds, such as acrylates, in equimolar amounts, leading to the complete reaction of the activated ethylenic double bonds. Systematic investigations into the addition of amino alcohols onto acrylates have been carried out by N. Ogata and T. Asahara, Bull. Chem. Soc. Jap. 39, pages 1486 to 1490, 1966.
DE-C 23 46 424 describes the preparation of radiation-curable compositions starting from acrylic esters of polyhydric alcohols and from secondary, aliphatic monofunctional amines.
U.S. Pat. Nos. 4,547,562 and 4,675,374 disclose radiation-curable solvent-free compositions based on polyacrylates and on mono-, di- or polyamines. Di- and polyamines mentioned as being suitable are species having more than one primary amino group. When these polyamines are used, highly crosslinked coating compositions of relatively high molecular mass and thus relatively high viscosity are obtained which without the addition of reactive diluents and solvents have an excessive application viscosity or even lead to solid products.
Finally, U.S. Pat. No. 4,045,416 and U.S. Pat. No. 3,845,056 relate to radiation-curable coating compositions based on amine acrylates, which are obtained by reacting polyacrylates with amines having at least one amine hydrogen. Amines mentioned as being suitable are primary and secondary monoamines and also polyamines. In accordance with the U.S. patents, the polyamine components specified are polyamines having more than one primary amino group, polyamines having exclusively secondary amino groups, or polyamines having secondary and tertiary amino groups.
Coating compositions according to the as yet unpublished German Patent Application 195 45 123.6 are radiation-curable without the use of external synergists, for example amines. The coating compositions described in the as yet unpublished German Patent Application 195 45 123.6 contain oligomers of low viscosity having comparatively good radiation-curability and lead to comparatively good properties in the coating film, especially with respect to the hardness of the coating film. The coating compositions are of low viscosity, so that it is possible to avoid the use of reactive diluents and organic solvents. Furthermore, the coatings produced with such coating compositions couple good sandability with good adhesion, even on difficult substrates.
A disadvantage for the use of all coating compositions of the prior art in UV-curable systems is the presence of low molecular mass UV initiators. Such low molecular mass UV initiators lead to fragments during the UV curing of the coating compositions, and these fragments lead in turn, firstly owing to the relatively high proportion of UV initiators in the coating composition, to notable losses of mass in the coating composition, and thus in the resulting coating, and, secondly, owing to the high volatility of the initiator fragments, lead to unwanted, strongly smelling and often toxic emissions in the course of curing of the coating compositions.
Copolymerizable UV initiators are known from the prior art with which it is possible to prevent the occurrence of low molecular mass initiator fragments. Included in the class of such copolymerizable UV initiators are benzophenonetetracarboxylic acid, its dianhydride and its esters, as are employed, for example, for heat-resistant photoresist materials as described, for example, in EP-A-0 379 377.
U.S. Pat. No. 4,100,047 embraces aqueous, UV-curable coating compositions comprising a hydroxy-functional ethylenically unsaturated polymer, for example an ether of polyethylene glycol and hydroxyethyl acrylate, benzophenonetetracarboxylic dianhydride as photo-initiator, which is attached to the hydroxy-functional ethylenically unsaturated polymer, and a further low molecular mass photoinitiator. The coatings prepared with coating compositions according to U.S. Pat. No. 4,100,047 can be cured in air at an intensity of irradiation of 2200 W/inch at belt speeds of about 3 m/min. A disadvantage of the coating compositions according to U.S. Pat. No. 4,100,047 is the high intensities of irradiation required at comparatively low curing rates.
U.S. Pat. No. 4,216,306 claims coating compositions comprising a polymeric adduct of diglycidyl ether, acrylic acid and benzophenonetetracarboxylic dianhydride, and a low molecular mass photoinitiator. Here too a disadvantage is the appearance of low molecular mass, volatile fragments which originate from the decomposition of the initiator. Furthermore, coatings prepared with coating compositions according to U.S. Pat. No. 4,216,306 must be cured in 3 passes at belt speeds of about 10 m/min, which points in turn to a comparatively low curing rate.
Finally, U.S. Pat. No. 4,338,171 describes coating compositions consisting of mixtures of monomeric ethylenically unsaturated compounds, for example trimethylolpropane triacrylate, and benzophenonetetracarboxylic diesters or tetraesters as photo-initiators, which, although they are highly reactive and can be cured at belt speeds of about 30 m/min, have the disadvantage of the high volatility of the monomeric component which makes up the predominant proportion of the coating composition and the high proportion of photoinitiator which is necessary.